1. Field of the Invention
This invention relates to a system for reading the recorded patterns of magnetically recorded information, which utilizes the magneto-optic effect.
2. Description of the Prior Art
The method of optically reading magnetically recorded information by the use of the magneto-optic Kerr effect is known, and particularly the method of reading recorded patterns which uses the polar Kerr effect from a vertical magnetic recording member has been widely used. An optical system as shown in FIG. 1A of the accompanying drawings is used for the electrical detection of such recorded patterns.
In FIG. 1A, reference numeral 2 designates a polarizing plate, reference numeral 3 denotes a half mirror, reference numeral 4 designates an objective lens, reference numeral 5 denotes a vertical magnetic recording member, reference numeral 6 designates an analyzer, reference numeral 7 denotes an eyepiece lens, and reference numeral 8 designates a photoelectric detector.
A light beam 1 becomes a light beam linearly polarized in a predetermined direction by the polarizing plate 2, and enters the vertical magnetic recording member 5. The half mirror used in the conventional system is of a transmission factor and reflection factor of approximately 50% independently of the direction of polarization. The plane of polarization of the light beam is subjected to rotations in opposite directions by the Kerr effect corresponding to the direction of magnetization (upward or downward) of the vertical magnetic recording member 5 and is reflected. For example, if the plane of polarization of the light beam reflected by a downwardly magnetized portion is subjected to rotation of .theta.K, the plane of polarization of the light beam reflected by an upwardly magnetized portion is subjected to rotation of -.theta.K.
Where the incident light beam is P-polarized as shown in FIG. 1B of the accompanying drawings, and if the direction of polarized-light transmission of the analyzer 6 is disposed in a perpendicular direction (Q-direction) with respect to the direction of polarization -.theta.K, the reflected light from the upwardly magnetized portion is intercepted by the analyzer 6, and the component .DELTA. of the reflected light from the downwardly magnetized portion transmitted through the analyzer 6 passes through the analyzer 6. By this phenomenon, vertically magnetized patterns can be detected.
However, the vertical magnetic recording member known heretofore is generally small in Kerr rotation angle .theta.k and a sufficient SN ratio cannot be obtained therefrom. Therefore, for example, a proposition as disclosed in U.S. patent application Ser. No. 382,202 has been made. In this proposition, a polarizing beam splitter is used instead of the half mirror of FIG. 1A and of the light beam reflected by the vertical magnetic recording member 5, as compared with the polarized component in said predetermined direction (the P-polarized component in the example of FIG. 1B), the polarized component in a direction perpendicular thereto (the S-polarized component of FIG. 1B) is increased to thereby apparently increase the Kerr rotation angle and enhance the SN ratio. Therefore, the characteristic of the polarizing beam splitter used in the aforementioned proposition is set so as to maximize the modulated component by the magneto-optic Kerr effect.
However, the SN ratio is varied under the influence of the noise in the photoelectric detector depending on the reflection factor of the magnetic recording medium or the construction of the photoelectric detector and, even if the characteristic of the polarizing beam splitter is set as shown in the aforementioned proposition, a maximum SN ratio cannot always be obtained.